feat: improve nav menu responsive layout, bump v2026.4.306}
- Redesign menu tabs container to use flexbox with proper alignment on desktop (768px+) and column layout on mobile (<767px) - Change nav pills from `nav-fill flex-nowrap` to `nav-justified` for better space distribution - Move flex/alignment styles from Twig inline classes to SCSS for cleaner separation of concerns - Adjust nav-link padding and font-size breakpoint from 576px to 991px for better tablet support - Update AGENTS.md with nav pills responsive behavior notes, dark/light theme guidance, and 29-language i18n info - Update CSS cache-busting hash across all theme files in themes.js - Bump version to v2026.4.306 in package.json, update build version to v2026.4.305 across docs